Intermolecular Forces of Attraction

Standards Students know the atoms and molecules in liquids move in a random pattern relative to one another because the intermolecular forces are too weak to hold the atoms or molecules in a solid form.

Intermolecular Forces Forces that attract molecules to other molecules. These include: Hydrogen bonding Dipole-dipole attraction London dispersion forces

Polarity A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment. H F + -

Electronegativity difference between 0.3 and 1.67 Polarity Polar-covalent bond – covalent bond in which the bonded atoms have an unequal attraction for the shared electrons. Electronegativity difference between 0.3 and 1.67 (Greater than 1.67 is ionic)

Relative Magnitudes of Forces The types of bonding forces vary in their strength as measured by average bond energy. Strongest Weakest Covalent bonds (400 kcal) Hydrogen bonding (12-16 kcal ) Dipole-dipole interactions (2-0.5 kcal) London forces (less than 1 kcal)

Hydrogen Bonding Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen Base pairing in DNA by hydrogen bonding

Hydrogen Bonding in Water

Dipole-Dipole Attraction Attraction between oppositely charged regions of neighboring molecules. Dipole-dipole attraction in hydrogen chloride, a gas that is used to make hydrochloric acid

London (Dispersion) Forces The weakest of intermolecular forces, these forces are proportional to the mass of the molecule These are the only forces of attraction between completely nonpolar molecules Large nonpolar molecules may have substantial dispersion forces, resulting in relatively high boiling points Small nonpolar molecules have weak dispersion forces and exist almost exclusively as gases

London Forces in Hydrocarbons